It is widely known that the stability of the knee joint depends primarily on the strength of the quadriceps muscle. It is widely recognized that strengthening and rehabilitation of the quadriceps muscle is best achieved by having a patient straighten his or her knee joint in opposition to a resistive force applied at the patient's instep. Such devices for exercising a quadriceps muscle are found, for example, in U.S. Pat. No. 3,120,954 (Apostol), U.S. Pat. No. 3,558,131 (Dragon), U.S. Pat. No. 4,254,949 (Brentham) and U.S. Pat. No. 4,304,401 (Goodman). The Apostol, Dragon and Brentham devices are relatively large and not readily moved from place to place. Thus, the Apostol and Dragon devices, which are used for exercising muscles other than the quadriceps, are practical only for use in the office of a physical therapist or in such other public exercise facility. The Brentham device, which takes the form of a chair with built in exercising apparatus, is practical for home use but is not sufficiently portable to permit it to be taken with the patient on a trip, or the like. The Goodman device is relatively portable in that it can be readily lifted in one hand and transported by a patient. However, in achieving the desirable portability feature Goodman has sacrificed tension adjustability and other considerations. Specifically, Goodman employs a planar surface supported at approximately chair height in front of a suitable chair. The patient sits in the chair with his leg extending over the planar surface so that the knee bends and the lower part of the leg is suspended. An elastomeric strap is secured to the patient's instep so that the patient may straighten his or her leg against the tension in the strap to exercise the quadriceps muscle. The tension in the strap is not adjustable so that the Goodman arrangement has limited use in view of the varying degrees of muscle strengthening and rehabilitation needs of a user. Moreover, the Goodman apparatus, in use, places relatively high stress on the patient's knee since the greatest tensile stress in the strap occurs when the gravitational force acting on the patient's leg is at a maximum (i.e., as the leg approaches its straightened position). Additional stress is placed on the knee by virtue of the fact that the effective pivot axis of the apparatus is considerably below the bending axis of the knee, resulting in strain on the knee rather than on the quadriceps muscle during the use of this device. In this regard, it should be noted that the only prior art known to me wherein the effective pivot axis of the exercise device is aligned with the bending axis of the knee is the Brentham device. However, in Brentham, as noted above, the apparatus is not sufficiently portable to permit it to be carried from place to place in one hand. In addition, the Brentham device lacks sufficient adjustability in the resistive force against which the patient is exercising. Still further, the Brentham device is designed to exercise both the quadriceps muscle and the hamstring muscle. This is achieved by utilizing an hydraulic element to resist both upward and downward movement of the lower portion of the leg. While a device for exercising both the quadriceps and hamstring muscles is desirable, the use of a single resistive element for both muscles in a single exercise routine presents severe calibration problems. More particularly, it is difficult to find a common resistive force which is suitable for exercising both the hamstring and quadriceps muscles.